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Phosphorus-Doped Hard Carbon Nanofibers Prepared by Electrospinning as an Anode in Sodium Ion Batteries Feng Wu, Ruiqi Dong, Ying Bai, Yu Li, Guanghai Chen, Zhaohua Wang, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018
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ACS Applied Materials & Interfaces
Phosphorus-Doped Hard Carbon Nanofibers Prepared by Electrospinning as an Anode in Sodium Ion Batteries Feng Wu,†,‡ Ruiqi Dong,† Ying Bai,*,† Yu Li,† GuangHai Chen,† Zhaohua Wang,† Chuan Wu*,†,‡ †
Beijing Key Laboratory of Environmental Science and Engineering, School of
Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡
Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR
China
KEY WORDS: electrospinning, phosphorus-doped, hard carbon nanofibers, electrochemical performances，sodium-ion batteries ABSTRACT: Phosphorus-doped hard carbon nanofibers with macroporous structure were successfully synthesized by electrospinning followed by thermal treatment process, using polyacrylonitrile and H3PO4 as carbon and phosphorus precursors, respectively. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the doped phosphorus atoms can incorporate into the carbon framework and most of them are connecting with carbon atoms to form P-C bond. The (002) plane interlayer spacing was taken from the X-ray diffraction (XRD) pattern, which shows a large spacing of 3.83 Å for the obtained P-doped hard carbon nanofibers. When used as an anode in sodium ion batteries, the as prepared P-doped hard carbon nanofibers can deliver a reversible capacity of 288 mAh g-1 and 103 mAh g-1 at a current density of 50 mA g-1 and 2 A g-1, respectively. After 200 cycles at 50 mA g-1, the capacity retention of P-doped hard carbon nanofibers still reaches 87.8%, demonstrating good cycling durability. These excellent electrochemical performances of P-doped hard carbon nanofibers can be attributed to the macroporous structure, large interlayer spacing and the formation of P-C bond.
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1. INTRODUCTION Energy storage and conversion technologies have been extensively studied over the past decades for increasing energy demand as well as the price of non-renewable fossil fuels.1 In recent years, large-scale energy storage technologies based on rechargeable batteries have become brilliantly promising in this background.2 Notably, among these diverse energy storage technologies, sodium ion batteries (SIBs) have obtained wide concern owing to not only high natural abundance and low price of sodium, but also its similar chemical characteristics compared with lithium.3 Over the past several years, various cathode materials for SIBs have been reported, such as layered oxides,4 prussian blue analogues5 and polyanionic compounds.6-8 However, an obstacle that shelved the research about SIBs was the lack of suitable anode materials until the use of hard carbon in 2000 by Stevens and Dahn, which has a reversible capacity up to 300 mAh g-1, closing to lithium storage in graphite.9 Since then, the researches of anode materials for SIBs have presented a trend of blowoutdevelopment. A large number of anode materials have been explored, for instance, carbonaceous materials (hard carbon,10-15 soft carbon,16-17 graphene18-19 and expanded graphite20), alloys, metal oxide/ sulfide/ phosphide21-22 and so on. Among the aforementioned anode materials, hard carbon has been mostly investigated due to its abundant resource, stability and nontoxicity. However, there is nearly a consensus that hard carbon has relatively poor performances in terms of its rate capability, reversible capacity and low initial Coulombic efficiency (ICE). To overcome these defects, many studies have focused on designing unique structures (microspherules,23-24 nanowires,25 nanofibers,26-28 porous structure
29-30
) or introducing heteroatoms to
elevate its electrochemical properties. In these studies, one-dimensional (1D) nanomaterials have attracted much attention on account of that the unique structure of them can provide short diffusion path for ionic and electron transport.31 In order to synthesize one dimensional (1D) structure materials for rechargeable batteries, the versatile and scalable electrospinning was adopted by many previous works.30, 32-42 For example, Yu et al.30 synthesized porous 2
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CNF (P-CNFs) electrodes via thermal treating PAN/Pluronic F127 nanofibers precursor and the specific capacity of the P-CNFs up to 266 mAh g-1 after 100 cycles at 50 mA g-1. Chen et al.32 fabricated carbon nanofibers (CFs) with polyacrylonitrile, when used as anode for SIBs, the CFs displayed a reversible capacity of 233 mAh g-1 at 50 mA g-1. Cao et al.40 synthesized porous carbon nanotubes (CNTs), by phasesegregation via electrospinning method. As using the porous CNTs in sodium-ion battery, it specific capacity retains 110 mAh g−1 over 1200 cycles at 5 A g−1. Doping covalent heteroatom, such as N, B, F, P and S is a valid path to improve sodium storage capacity.43-47 So far, N is the most adopted heteroatom, which can enhance the reaction activity and electronic conductivity by forming extrinsic defects in.43 The effects of F-doping and B-doping are similar to N-doping. However, these heteroatom-doping mainly enhance the slope capacity rather than the plateau capacity. S-doping carbon anodes generally have high voltage plateau, thus limiting the practical utilization.47 Therefore, the P-doped hard carbon is regarded as a promising candidate anode material because of the introduction of blistering in its structure, which can facilitate adsorbing/inserting more Na+ ions.48 Recently, Ji et al 49 prepared a POx-doped hard carbon anode material, and enabled the capacity to increase from 283 to 359 mAh g-1. Wu et al.50 synthesized a flexible P-doped carbon cloth, when directly used as anodes for SIBs, it can deliver a reversible capacity up to 242.4 mAh g-1 at 50 mA g-1. Wang et al.48 designed a phosphorus-doped graphene anode, and it could hold an ultrahigh capacity of 374 mAh g-1 after 120 cycles at 25 mA g-1. Unlike these works which using red phosphorus as phosphorus precursors through solid phase blending to prepare P-doped carbon, or using graphene as carbon precursors which possess large specific surface area leading to forming excess solid electrolyte interface (SEI). In this work, we chose H3PO4 as the phosphorus source, and adopted the facile electrospinning approach to prepare P-doped 1D macroporous nanofibers with low specific surface. The homogenous doping of P was guaranteed through the means of liquid phase mixing. When served as an anode for sodium ion batteries, the obtained hard carbon nanofibers show excellent electrochemical 3
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performances with a reversible capacity of 288 mAh g-1 and 103 mAh g-1 at a current density of 50 mA g-1 and 2 A g-1, respectively. The homogenous distribution of P and the unique 1D nanofibers with macropores morphology are expected to significantly contribute for the improvement of the electrochemical performances. 2
EXPERIMENTAL SECTION
2.1. Synthesis of P-doped hard carbon nanofibers. Typically，the spinning solution was prepared by dissolving 1 g polyacrylonitrile (PAN, average Mw=150 000, Aldrich) in 9 g N, N-dimethylform amide (DMF), then 0.15 g H3PO4 (15% weight of PAN) was added into the mixture, then stirred at 40 oC for 12 h to well mixed. Next, the obtained solution was transferred into a 10 mL syringe linking with a stainless-steel tip needle. Then the syringe was fixed with the needle tip is 17 cm from the collector. Electrospinning was performed by applying high voltage of +18kV and -3kV to the needle and the Al foil collector, respectively. The injection rate of the syringe was controlled as 1 ml h-1. Afterwards, the asprepared nanofibers were stabilized in a muffle furnace at 260 oC keeping for 2 h. Finally, the hard carbon nanofibers were obtained via thermal treating the stabilized nanofibers at 1000 oC for 5 h under Ar atmosphere as shown in Scheme 1. The Pdoped hard carbon nanofibers were denoted as CFs-P15. For comparison, the hard carbon nanofibers without P-doping were prepared following the same method without using H3PO4 and named as CFs. 2.2. Material Characterization. The P-doped content in CFs-P15 was detected using inductively coupled plasma optical emission spectrometry (ICP-OES), and it was 1.4 wt%. The morphologies of the samples were obtained by HRTEM (HITACHI H-800) and SEM (ZEISSSUPRA55) with energy dispersive X-ray Spectrometer mapping (EDS). To characterize the structures of the samples, X-ray diffraction (XRD, Rigaku DMAX2400) analysis was carried out using Cu Kα radiation (40kV, 40mA, 8°/min from 10 to 80°), and Laser Raman spectroscopy (LRS, Renishaw inVia) was conducted with a wavelength of 633 nm. The specific surface area and pore size 4
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distribution of the samples were investigated by carbon dioxide adsorption measurements (ASAP 2020, Micromeritics) at 273K. XPS measurements were carried out on a PHI QUANTERA-II SXM system to survey the chemical composition on the surface of CFs-P15. 2.3. Electrochemical Measurements. The working electrodes were prepared with hard carbon, polyvinylidene fluoride (PVDF) and acetylene black at.a mass ratio of 80:10:10 Through homogenously mixing all of these components in N-methylpyrrolidone solvent, a slurry was obtained. Then, the slurry was spread on a Cu foil with thickness of 75 µm and dried under vacuum at 80 °C for 12 h. Afterwards, coin cells (CR2035) were assembled in a glove box, filled by high-pure argon atmosphere (MBRAUN-6020, H2O, O2
